In recent years in the field of wireless communications, various services, such as Internet accesses, distribution of streaming broadcast, distribution of contents (e.g., music and videos), have been provided, in addition to voice calls. An increase in the communication speeds and wider connection have created needs for provision of such services in higher qualities anywhere.
For enhancing the communication speeds, the Long Term Evolution (LTE) standard has been stipulated by the 3rd Generation Partnership Project (3GPP), one of the standardization organization, and has been put into service. The LTE scheme achieves maximum communication speeds of about 300 Mbps for downlinks and about 150 Mbps for uplinks.
For enhancing the connection areas, in the LTE-Advanced (LTE-A) which is the next-generation standard of the LTE, introduction of a relay station that relays wireless signals sent and received between user equipments and a base station has been discussed.
Note that in a communication system compliant with the LTE scheme, a technique has been proposed to ensure the validity of a reconnection request to a base station when a user equipment detects a radio link failure (RLF) in a radio resource control (RRC) connection (refer to the following Patent Reference 1).
Patent Reference 1: Japanese Laid-open Patent Publication No. 2010-4503
For example, if a user equipment moves and the wireless propagation environment around the user equipment varies accordingly, the reception quality at the user equipment may deteriorate. Since the connection with the base station may become unstable due to any detonation of the reception quality, a user equipment monitors the status of the connection with base station, based on the reception quality of wireless signals.
If the reception quality deteriorates to the level hindering the wireless communication from being continued and if the status of the reception quality does not recover within a certain time duration, the user equipment detects that an RLF occurs and executes reconnection processing to switch connections to another base station, another cell, another sector, or the like.
Here, an example of the reconnection processing by a user equipment is depicted in FIG. 1.
As depicted in FIG. 1, once a user equipment establishes a connection with a base station in a predetermined connection sequence and initiates a communication (Step S 1), the user equipment measures the reception quality of wireless signals on a link (access link) between the user equipment and the base station at regular or irregular intervals (Step S 2).
The user equipment then compares the measured reception quality against a predetermined threshold level, and detects that a link error occurs if the reception quality is smaller than the predetermined threshold level (Step S 3).
Then, if no link error is detected (the NO route from Step S 3), the user equipment repeats the processing in Steps S 2 and S 3. In contrast, if a link error is detected (the YES route from Step S 3), the user equipment starts s timer (status recovery monitoring timer), for monitoring whether or not link error status is not recovered within a certain time (Step S 4).
The user equipment then monitors whether or not the link error status on the access link recovers (Step S 5). For example, the user equipment compares the measured reception quality against a predetermined threshold level, and monitors whether or not the reception quality is equal to or greater than the predetermined threshold level.
The user equipment then determines whether or not the status of the access link recovers (Step S 6). If the status of the access link recovers (the YES route from Step S 6), the status recovery monitoring timer is stopped (Step S 7) and the flow transitions to Step S 2. In contrast, if the status of the access link does not recover (the NO route from Step S 6), the user equipment determines whether or not the status recovery monitoring timer times out (Step S 8).
If the status recovery monitoring timer does not time out (the NO route from Step S 8), the user equipment repeats the processing in Steps S 5, S 6 and S 8. In contrast, if the status recovery monitoring timer times out (the YES route from Step S 8), the user equipment detects an occurrence of an RLF and executes reconnection processing to switch the connection to another base station, another cell, another sector, or the like (Step S 9).
The user equipment then determines whether or not reconnection processing succeeds (Step S 10). If it is determined that the reconnection processing succeeds (the YES route from Step S 10), the flow transitions to Step S 2 to transition to the normal state. In contrast, if it is determined that the reconnection processing fails (the NO route from Step S 10), call disconnection processing is performed to terminate the communication with the base station (Step S 11). Note that the user equipment that terminates the communication transitions to the reception standby status where an execution of a connection sequence with the base station is waited, for example.
Here, the timeout value of the status recovery monitoring timer (hereinafter, also simply referred to as “timer value”) is determined based on the quality of service (QoS) of the service used by the user equipment.
For example, if the user equipment is using a voice communication service, the time until an RLF is detected becomes longer and silence or noises are generated as the timer value is increased, which may annoy the user. Hence, in the case of the voice communication service, the timer value is set to a smaller value.
In contrast, if the user equipment is using a communication service, such as packet communication call, of non real time communications (e.g., web browsing), a certain delay is permissible as long as the communication is available. Hence, the timer value is set to a value longer than the value used for the voice communication service.
As examples of the timer values for the Universal Mobile Telecommunications System (UMTS), the 3GPP TS25.331 stipulates T314 as the timer value for circuit switching calls, such as voice calls, and T315 as the timer value for packet exchange calls. The initial values of T314 and T315 are 12 seconds and 180 seconds, respectively.
However, documents, such as Patent Reference 1 described above, do not specifically propose how a relay station used in a LTE-A communication system executes RLF detect processing and reconnection processing on a link (relay link) between a base station and the relay station. Hence, effective wireless communication technique cannot be provided in such a wireless communication system.